The hardware is commodity, but you've centralized the software/logic. Is this really a net win on a global scale? (It's obviously a net win at the micro level, or else Google wouldn't use it)
[1] http://static.usenix.org/event/inm10/tech/full_papers/Tootoo...
End result is that nearly a decade later, physically installing more than 3 servers at any given time is a project that takes many weeks. In one instance, installation of a small cluster took over 3 years.